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Bias dependence of spin injection/transport
properties of a perpendicularly magnetized
FePt/MgO/GaAs structure
To cite this article: Rento Ohsugi et al 2016 Appl. Phys. Express 9 043002

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Applied Physics Express 9, 043002 (2016)
http://doi.org/10.7567/APEX.9.043002

Bias dependence of spin injection/transport propertiesof a perpendicularly magnetized FePt/MgO/GaAs structure
Rento Ohsugi1*, Yoji Kunihashi2, Haruki Sanada2, Makoto Kohda1, Hideki Gotoh2, Tetsuomi Sogawa2, and Junsaku Nitta1*
1

Department of Materials Science, Tohoku University, Sendai 980-8579, JapanNTT Basic Research Laboratories, Atsugi, Kanagawa 243-0198, Japan2*E-mail: ohsugi@dc.tohoku.ac.jp
Received January 28, 2016; accepted February 15, 2016; published online March 2, 2016
We demonstrate injection and transport of perpendicularly spin-polarized electrons in an FePt/MgO/n-GaAs structure. Spin-polarized electrons
were injected from a perpendicularly magnetized FePt layer into an n-GaAs layer through a MgO barrier and detected by spatially resolved Kerr
rotation microscopy. By measuring the Hanle effect, we reveal that the injected/extracted spin polarizations drastically vary with bias voltages. A
spin lifetime of 3.5 ns is obtained that is consistent with the result from pump–probe measurements. This direct observation of perpendicularly
polarized spin injection and lateral transport is one step toward realizing future spintronic devices. © 2016 The Japan Society of Applied Physics

lectrical spin injection into semiconductors is essential for the realization of active spintronic devices
such as a spin ﬁeld eﬀect transistor (FET),1,2) which is
expected to provide reconﬁgurable and low power logic
systems. The spin FET can manipulate injected spins by anin-plane eﬀective magnetic ﬁeld caused by the spin–orbit
interaction (SOI) in III–V semiconductors.3) The strength of
the SOI can be controlled by a gate voltage,4) resulting in agate-controlled spin precession.5) Koo et al. have investigatedspin FET operation using an in-plane magnetized spin
injector,6) while the perpendicularly magnetized spin injectors are preferable for eﬃcient spin manipulation by thein-plane eﬀective magnetic ﬁeld.7) Moreover, a perpendicularmagnetized ferromagnet can reduce the contact size morethan an in-plane one due to strong uniaxial anisotropy.
Therefore, the perpendicularly polarized spin injector is apromising candidate for future spintronic devices. Recently,
several groups have reported perpendicular spin injection
by using spin LED structures8–12) and three-terminal electrical Hanle measurements.13) These methods only allow for
the detection of longitudinal spin transport and interface
spin accumulation, respectively, although the lateral spintransport is essential for logic and integrated circuits. In thiswork, we investigate the detection of lateral spin transport
for perpendicularly oriented spins by spatially resolved
Kerr rotation microscopy that enabled us to directly observe
the spatial distribution of injected spins.14) Furthermore,
the method accurately determined the spin polarization
without any spurious contributions that are observed inelectrical methods. We used an FePt=MgO=GaAs structure
to demonstrate electrical injection of the perpendicularly
oriented spins and the lateral spin transport. The FePt has
a strong uniaxial magnetic anisotropy perpendicular to theﬁlm plane15) that is stable even when subjugated to thermaland electrical stress. The MgO serves not only as a buﬀer
layer for the FePt, but also as a tunnel barrier that overcomesthe conductivity mismatch problem of spin injection between
the ferromagnetic metal and semiconductor.16,17) Since
injected=extracted (in=ex) spin polarization and lifetime canbe aﬀected by materials and applied bias voltages,18) weevaluated the bias voltage dependence of spin injection andlateral transport properties in the FePt=MgO=GaAs structureE

at 5 K. We measured Kerr rotation signals caused by the
injected spins in the GaAs channel, and extracted bias voltage
dependencies of the in=ex spin polarizations and the spin
lifetimes.
We used an n+-GaAs (20 nm) highly doped layer=n-GaAs(2 µm) channel grown on an insulating GaAs substrate bymolecular beam epitaxy. Doping concentrations of the highly
doped and channel layers were 2 × 1019 and 3 × 1016 cm−3,
respectively. The highly doped layer reduces the Schottky
barrier width between MgO and GaAs to eﬀectively inject
spins. Prior to the sputtering of FePt=MgO, the GaAs surface
was cleaned by a HCl : H2 O ¼ 1 : 1 liquid solution for
1 min and ðNH4 Þ2 S : H2 O ¼ 1 : 1 liquid solution for 1 min toremove the oxide layer and suppress its growth on the GaAs
surface. The samples were immediately transferred into an
ultra-high vacuum chamber to avoid surface oxidation andannealed at 400 °C for 25 min. The wafer was then examinedby reﬂective high-energy electron diﬀraction (RHEED) thatshowed clear 1 × 2 streak patterns along the 〈110〉 and 〈100〉directions, indicating a ﬂat and clean surface. A MgO (1 nm)
tunnel barrier was deposited by radio frequency (RF) magnetron sputtering with an Ar pressure of 0.8 Pa and a substrate temperature of 300 °C. After the MgO deposition, thesample was heated to 400 °C and an Fe43Pt57 (20 nm) layer
was deposited by a co-sputtering process at an Ar pressure of
0.8 Pa. The crystal structures of L10-FePt=MgO on n-GaAs
were determined by X-ray diﬀraction (XRD) and RHEED.19)
XRD patterns show the FePt (001), (002), and (003) peaks
indicating a perpendicularly oriented c-axis of a face-centered
tetragonal structure on a MgO=n-GaAs structure. RHEED
clearly shows streak patterns for the MgO and FePt surfaces.
As a result, we conﬁrmed epitaxial growth of L10-FePt=MgO
on the n-GaAs channel layer. The ratio of remanent andsaturated magnetizations of the FePt layer was 0.98 at room
temperature as obtained by polar magneto-optical Kerr rotation measurements shown in Fig. 1(b). Photolithography
and electron beam evaporation were used to fabricate a spin
injection device shown in Fig. 1(a) where the contact 1, 2,
and 3 are the ohmic reference (C1), spin injector (C2),and drain electrode (C3), respectively. In order to eliminate
parallel conduction between the n-GaAs channel andhighly doped n+-GaAs layer, we removed the highly doped

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043002-1© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 043002 (2016)(a)R. Ohsugi et al.
(b)

(c)

Fig. 1. (a) Device and schematic of measurement conﬁguration. C2 is the FePt=MgO spin injector. C1 and C3 are the ohmic drain electrode. The enlarged
area is optically detected domain of the GaAs channel. A red dot is the probe beam position located in the GaAs channel at 5 µm away from the spin injector(C2). Magnetic ﬁelds were applied in y-direction during optical measurements. Two-terminal voltages (V2T) contain both channel and contact voltage drops.
The three-terminal connection (V3T) only provides a contact voltage of the spin injector (C2). (b) Polar Kerr signal of perpendicularly magnetized spin injector(FePt=MgO) at room temperature. (c) I–V characteristics of spin injector C2 at 5 K. Blue (V3T) and red (V2T) lines show I–V measured in the conﬁgurationof (a).

Measured at 5 K
Bias voltage
V2T

Kerr rotation (arb. unit)

n+-GaAs layer by a wet etching process using a H2 O :H2 O2 : H3 PO4 ¼ 38 : 1 : 1 solution. The width of the GaAs
channel is 60 µm, and the contact area of the spin injector
is 60 × 60 µm2. Figure 1(c) shows two- and three-terminal
current–voltage (I–V ) signals obtained by measuring voltages
V2T between C2 and C3, and V3T between C2 and C1.
From the measurements, we separated these applied bias
voltages at the tunnel barrier and the GaAs channel. In
the reverse bias conﬁguration, electron spins are injectedfrom the ferromagnetic FePt electrode to the GaAs channel.
Meanwhile, under forward bias conditions, electron spinsare accumulated in the GaAs channel. The resistance-area
product at a bias voltage of −1.0 V is 17.8 × 10−5 Ω m2.
Samples were inserted into a cryostat with optical windows
and cooled down. All measurements were performed at
5 K. In the spin injection and optical probe measurements,
the spins were injected and extracted by applying a DCvoltage source with an AC voltage of 55.3 kHz, and spins
were detected with the probe beam 5 µm away from anedge of the spin injector. The probe spot is shown as a red
dot in the enlarged image in Fig. 1(a). The probe beam
was modulated by an AOM at a frequency of 52 kHz.
The diameter of the probe beam was ∼2–4 µm. A lock-in
ampliﬁer was synchronized at a diﬀerential frequency of
3.3 kHz to detect the Kerr signals. To observe the spin
precession, a magnetic ﬁeld was applied in the y-direction up
to a strength of ±0.2 T during the measurements as seen in
Fig. 1(a). The applied in-plane ﬁeld was kept so as not torotate the FePt magnetization.
Figure 2 shows the magnetic ﬁeld dependence of the Kerr
rotation signals with spin injection=extraction as a function ofbias voltages from −1.8 to +1.8 V. A 300 µW probe power
was used with an energy of 1.514 eV. Injected spins wereprobed at a position of 5 µm away from the edge of the spin
injector. The observed signals in Fig. 2 show spin dephasing
due to the spin precession by the magnetic ﬁeld, i.e., theHanle eﬀect (Hanle signals). The sign of the Hanle signals
at zero magnetic ﬁeld is ﬂipped with the bias voltage,
which corresponds to the averaged direction of injected spins.
Thus, the sign change of the polarity means opposite spin
accumulation in the GaAs channel.

+1.8

−1.8
-0.2

-0.1

0.0

0.1

0.2

Magnetic field By (T)Fig. 2. Magnetic ﬁeld dependence of Kerr rotation signals with spin
injection=extraction as a function of bias voltages (V2T). From top(V2T = 1.8 V) to bottom (V2T = −1.8 V) with voltage step of −0.4 V.
The probe beam was focused at 5 µm away from the edge of spin injector[C2 in Fig. 1(a)]. The circle and triangle signals correspond to those of spin
extraction and injection regimes, respectively. Solid lines are the ﬁtted curves
by Eq. (1). All measurements were performed at 5 K.

The Hanle signals can be described by a spin dynamic
equation consisting of spin precession, relaxation, and driftdiﬀusion20,21) as follows:


Z1
S0
ðy  vd tÞ2
p
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
Sz ðy; By Þ ¼
exp 
4Dt
4Dt
0


t
 exp ð1Þ
cosð!L tÞ dt;
s
S0 ¼ Adin Ip  2spot ;ð2Þ
The position y = 0 means just under the edge of the spin
injector, S0 is the spin z-component at y = 0, vd is the drift
velocity of spin, τs is the spin lifetime, D is the spin diﬀusion
pﬃﬃﬃﬃﬃﬃﬃﬃ
constant which is described by a relationship LS ¼ DS ,where the LS is the spin diﬀusion length, and ωL is the Lamor
frequency written as !L ¼ gB By =ħ. g is the g-factor, −0.44,
for bulk GaAs. A is a constant, d is the thickness of theepitaxial ﬁlm, Ip is the intensity of the probe beam, σspot is thediameter of the probe beam, and ρin is the spin component pervolume. ρin is proportional to the spin polarization in GaAs.

043002-2© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 043002 (2016)(a)R. Ohsugi et al.

(b)

(a)

(b)

Fig. 3. Obtained spin injection=extraction parameters S0 and τs by the
Hanle ﬁttings of the curves in Fig. 2. (a) Spin polarization S0 as a function of
applied contact bias voltage V3T. The spin polarizations are plotted by thearbitrary unit. (b) Spin lifetimes τs vs electric ﬁelds in the n-GaAs channel.

To reduce ﬁtting parameters, we used LS = 10.2 µm obtained
from spatially resolved pump–probe measurements shownin Fig. 4(b). As can be seen in Fig. 2, the ﬁtting curves (solidline) reproduce the experimental results. By using Eq. (1),we extracted bias voltage dependencies of S0 and τs. Since thespin component S0 as shown in Eq. (2) corresponds toin=ex spin polarization in the optical detection, we evaluate
the bias voltage dependence of S0 as that of the in=ex spinpolarization.
Figure 3(a) shows the dependence of in=ex spin polarization, S0, on the contact bias voltage, evaluated from theHanle signals. An asymmetric dependence with the contact
bias voltage is evident. With the forward bias voltages,
the spin polarization monotonically increases with the bias
voltages above +0.6 V, while it reverses polarity with bias
voltages below +0.6 V, and peaks around −1.0 V. This canbe explained as majority and minority spin accumulations
in reverse and forward biasing, respectively, due to thespin dependent tunneling through the FePt=MgO=GaAs
interfaces. Spin polarization in=ex into the n-GaAs channel
is changed by the bias voltages depending on the spin
dependent density of states in the FePt layer and the bias
voltage applied at the FePt=MgO=GaAs interfaces. Therefore, the averaged in=ex spin polarization has asymmetricdependence on the bias voltage. Similar bias dependence
has been reported in other materials.18,22,23) At a bias voltage
of +0.6 V, extracted spins have no polarizations. The mosteﬃcient spin injection was performed at a reverse bias of
about −1.0 V.
Figure 3(b) shows spin lifetimes as a function of electricﬁeld. The spin lifetime of 3.5 ± 1.0 ns slightly depends on thebias voltages. The obtained spin lifetime is close to the spin
lifetime of 6.5 ns extracted from time-resolved Kerr rotation
measurement as shown in Fig. 4(a). It indicates that the Hanle
signals come from the perpendicularly polarized spin injection
into the n-GaAs channel. The diﬀerence between the spin
lifetime found using the Hanle and time-resolved Kerr
rotation measurements is because of the diﬀerence between
the transverse and longitudinal spin lifetime obtained fromHanle and time-resolved Kerr rotation, respectively. In thelightly doped n-GaAs (1016 cm−3), it is reported that thetransverse spin lifetime is decreased by magnetic ﬁelds andbecomes smaller than the longitudinal spin lifetime.24)
To conﬁrm a spin lifetime and a diﬀusion length in the
n-GaAs channel, we independently measured these properties(c)Fig. 4. Both time- and spatial-resolved Kerr rotation signals generated by
the optically excited spin polarization in the n-GaAs channel. The photon
energy of pump and probe beam were settled on 1.514 eV, and the powers of
those were 7 and 0.5 µW. The measurement temperature was 5 K. (a) Timeevolution of Kerr rotation signal. Kerr rotation signals that are proportional tospin polarization were decreased during pump–probe delay due to the spin
relaxation. The solid line is exponential ﬁtting with the delay time. (b) Spatial
resolved Kerr rotation signals with electric ﬁelds 0, 9.5, and 12 V=cm. The
solid lines show spatial decay of signals. Pumped spins at y = 0 diﬀuse
isotropically and drift downstream due to the electrical ﬁeld. Here, y > 0 are
the downstream side for electrons. (c) Calculated spin drift (Ld )-diﬀusion (Ls)lengths obtained by the exponential ﬁttings of the spatial resolved Kerrsignals [solid lines in (b)]. Red and blue symbols correspond to those ofelectric ﬁeld dependences downstream and upstream from pump beam
position (y = 0 µm). Ls is the spin drift-diﬀusion length at E = 0.

by means of time and spatially resolved Kerr rotation measurements with optically pumped spins in the n-GaAs channel
at 5 K. Figure 4(a) shows the time-resolved Kerr rotation
signals as a function of pump–probe delay times. The probe
beam energy was tuned to 1.514 eV, which is the same value
used in the Hanle measurement. The intensity of the pumpand probe beams was set to 7 and 0.5 µW, respectively. The
Kerr rotation signal exponentially decreases with the delaytime, which is ﬁtted by the equation: K ¼ 0 expðt=S Þ,
where θ0 is the initial Kerr rotation signal at the spin injection
point ðy ¼ 0; t ¼ 0Þ. An extracted spin lifetime τs = 6.5 ns
becomes comparable to the lifetime obtained from Hanle
measurements. This experimental value of spin lifetimeshows that the Hanle signals arise from electrical spin injection and transport in the n-GaAs channel (Fig. 2). Figure 4(b)shows Kerr rotation signals plotted against the y-distance
between the position of the spin pumping beam and the043002-3© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 043002 (2016)

R. Ohsugi et al.

scanning probe beam. The spin pumping position was ﬁxed
at y = 0. We applied electric ﬁelds with strengths of 0, 9.5,
and 12 V=cm to measure LS as a function of electric ﬁelds. In
Fig. 4(b), electrons ﬂow in the +y-direction. The asymmetricspin distributions with distance suggest an additional spin
drift eﬀect to spin diﬀusion due to the ﬁnite electric ﬁelds.
The spin drift diﬀusion lengths with various electric ﬁelds
were determined through the exponential ﬁtting of the plottedvalues in Fig. 4(b) with the equations:25,26)yK ¼ 0 exp ;ð3Þ
Ld
Ld ¼ s E;

ð4Þ

where Ld is the spin drift length. μ is the mobility of spin,and E is the electric ﬁeld. Here, under a strong electric
ﬁeld, Ld has a linear relationship with the mobility of spin
and lifetime. In Fig. 4(c), the spin diﬀusion length can bedeﬁned as a spin drift-diﬀusion length at zero bias voltage
where we can eliminate the spin drift eﬀect. A pure spin
diﬀusion length of 10.2 µm was obtained in the n-GaAs at
5 K. In addition, the observed linear electric ﬁeld dependenceof Ld, as shown in Fig. 4(c), experimentally illustrates theconstant spin lifetime as a function of bias voltage, consistent
with Fig. 3(b).
In conclusion, we have observed Hanle signals in an FePt=
MgO=GaAs structure at 5 K. The in=ex spin polarizations
drastically change with bias voltages, while the spin lifetime
only slightly depends on the bias voltages. The obtained spin
lifetimes are ∼3.5 ± 1.0 ns, comparable to the spin lifetimein the n-GaAs channel independently obtained by time andspatially resolved Kerr rotation measurements. These results
clearly demonstrate the perpendicularly oriented spin injection and its transport in the FePt=MgO=GaAs structure. This
structure could be a promising candidate for the realizationof future spintronic devices using perpendicularly polarized
spin injection.
Acknowledgments This work was supported by Grants-in-Aid for
Scientiﬁc Research (15H05699, 15H02099, and 25220604) from the Japan
Society for the Promotion of Science and by the ImPACT Program of Council for
Science, Technology, and Innovation. JN acknowledges partial support from theStrategic Japanese–German Joint Research Program.

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043002-4© 2016 The Japan Society of Applied Physics